Method and apparatus for reducing particulates in a plasma tool through steady state flows

ABSTRACT

A method and apparatus for reducing particulates in a plasma tool using steady state flows includes a device, operatively coupled to a housing in which an object to be processed is positioned, for generating a plasma flow adjacent the object toward a pumping aperture. A pumping mechanism pumps a medium adjacent the object. The medium supports the plasma and entrains particulates in the plasma away from the object and out the pumping aperture. Magnetic fields, produced by multipole magnets forming a ring cusp, are preferably used to produce the plasma flow which is directed radially away from the object to be processed. In a second embodiment, an array of magnets which form a line cusp is provided to produce an opening through which plasma will flow.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to a method and apparatus forreducing particulate contamination of semiconductor wafers during plasmaprocessing and, more particularly, to removing particulates in a plasmatool through steady state flows and to a system for removingparticulates in a plasma using a multipole magnetic field. The magneticfield produces a plasma flow and continues the plasma flow through anystagnation point, towards an opening in the magnetic field, to dragentrained particulates through the opening for removal from the system.With the invention, particulates are reduced in a plasma (and thus theobject being processed) by generation of the steady plasma flow.

Description of the Related Art

Semiconductor wafers, typically made of silicon (Si), are subjected toseveral processes during the manufacture of integrated circuits (ICs).Some of these processes involve a plasma (e.g., used in etching, plasmadeposition and sputtering). Radio frequency (RF) and direct charge (DC)Glow discharge plasmas, for example, are extensively utilized in themanufacture of ICs. Both electropositive and electronegative gases areused to produce plasmas. Electronegative gases (i.e., those having ahigher ion density than an electron density) such as CF₄, CHF₃, Cl₂, HBror O₂, present difficult contamination problems for semiconductormanufacturers.

Contaminants (e.g., contaminating particulates) ranging in size fromless than tenths of microns to several microns are produced or grown inthe plasmas or "pulled in" (e.g., assimilated) from adjacent surfaces ofa processing chamber or the object itself. The particulates normallyhave a negative charge, and are produced, for example, by negative ionswhich are trapped in the plasma by plasma sheaths. Plasma sheaths formwhere the plasma comes in contact with a solid and act as a boundarylayer between the plasma and the solid surface. The size of the sheathis inversely related to plasma density.

Increasing densities of semiconductor circuits make contamination aserious problem to product reliability and may even be a barrier toachieving theoretically possible higher density circuits. Contaminationby particulates also is a severe problem in the processing ofsemiconductor devices such as semiconductor wafers or the like. Indeed,for proper fabrication and processing of the semiconductor devices, thedevices must be reliably cleaned of particulates to a size ofapproximately one-tenth (0.1) of a micron.

Particulate contamination is a major problem encountered during plasmaprocessing of microelectronic materials. By some accounts, 50% ofcurrent semiconductor chip yield loss may be attributed to direct orindirect effects of particulate contamination during fabrication. Thisfraction is expected to increase as device dimensions are reduced infuture technologies. Particles that reduce process yields today range insize from the macroscopic to the sub-micron size.

Particulate contamination also has an extremely deleterious effect onthe performance and reliability of microelectronic devices produced byplasma etching or deposition. Particulate contamination can result indevice failure, poor film quality, changes in material resistivity, andimpurity permeation. Further, as device dimensions are reduced, tightercontrol of the etching profile requires ever more stringent restrictionson the allowable particle contamination number, density, and size. Tomeet these requirements, tightly controlled, clean rooms are required toavoid particle deposition on product surfaces during wafer transport andhandling.

Improvements in clean room technology and in the handling of in-processsubstrates (for semiconductors and other applications) have reduced theonce appreciable introduction of particulates onto substrates duringnon-process exposure such as wafer handling and transfer. Particulateformation during process steps, including plasma processing, may nowcontribute a significant fraction of total contamination exposure withcorresponding yield reduction.

Additionally, the industry trend is towards integrated vacuumprocessing", or "multi-channel processing". Thus, surface contaminationpreviously removed by wet or dry mechanical means will be more complexor impossible to remedy since it now requires removal of the substratefrom the vacuum chamber. In multi-chamber tools, particulates which droponto a wafer before, during, or at the completion of a process step mayhave an especially severe impact on subsequent process steps in thattool.

Recent studies have shown that certain etching plasmas can produceparticulates which may be a significant source of product contaminationand device failure. These experiments have shown that particles can benucleated, grown, and suspended in a process plasma until they aresignificant in size. For example, particles are formed with sizes on theorder of the submicron scale to hundreds of microns in diameter. Theparticles may ultimately fall onto devices being fabricated in the samemanufacturing environment. If particles fall before or during filmdeposition or pattern transfer, then they can disrupt the process step.If the particulates fall at the end of a process step, the particulatesmay disrupt subsequent process steps. These contaminants often producedefects which affect the device yield, performance and reliability.Similar results have been observed in deposition type plasmas (e.g.,PECVD Silane).

The effects of particulate contamination can be magnified when selectiveplasma etching processes are used. Certain plasma etching processes relyon a combination of feed gases and etching conditions to etch materialsurfaces on the wafer selectively. The chemical deformation ofparticulates which are etched at a slow rate in these highly selectiveplasmas results in micromasking, or an irregular surface often referredto as "grass". This spike or hill of unetched material will also degradethe device performance and reduce process yield.

The presence of these particulates is not always due to material flakingfrom chamber walls, but may also be due to gas phase processes such ashomogeneous nucleation. This suggests that particle contaminationproblems may not be eliminated solely by rigorous attention to cleanroom techniques and frequent cleaning of manufacturing equipment.Instead, since the plasma itself can result in product contamination,this problem may pose a "base level" of contamination even with thehighest clean room technology.

In a normal plasma, sheaths are formed with electric fields on the orderof 100 V/cm. These sheaths form to reduce diffusion of very mobileelectrons which have energies 5 to 10 times greater than that of theheavier ions in the plasma. Since the field in the sheaths retard thenegative electrons, the low energy negatively charged particles aretrapped in the plasma. As these particulates reach micron size, the iondrag forces push these particulates toward the plasma sheaths were theyremain under normal plasma conditions. When plasma undergoes transitionsin density caused, for example, by variations in the generator power,microarcs, or the plasma being turned off, these particulates can fallor are pulled to the wafer surface and adhere thereto.

In plasmas with magnetic surface confinement, i.e., multipole or cuspplasmas, the plasma is also bounded by the magnetic fields. In theseplasmas shown in FIG. 1(b), the plasma is bounded by the magnetic fieldto a region which is slightly larger than the wafer being processed. Theplasma is bounded by sheaths which prevent the particulates from leavingthe plasma except during perturbations of the plasma density aspreviously described.

Further, it has been found that small variations in the surface adjacentto the plasma causes "particle traps" such as a small trench and a step.These surface variations can have a size as small as a few millimetersand particulates will be trapped in the sheath above these surfacevariations. Even the edge of the wafer being processed can causeparticles to be trapped at the wafer's edge. Additionally, if the plasmadensity is slightly reduced above the center of the wafer, particleswill be trapped above the wafer, thereby falling or being pulled towardthe wafer when the plasma is turned off.

In view of the foregoing, it is therefore necessary to provide a way toreduce particle contamination on semiconductor wafers during plasmaprocessing with a plasma tool.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand apparatus for reducing particle contamination on wafers which mayoccur during plasma processing.

A second object of the invention is to provide an improved method andapparatus for reducing particulates in a plasma through generation ofsteady state flows.

It is a third object of the invention to remove particulates in a plasmaby using a multipole magnetic field for entrainment of the plasma andgas flow and evacuation of particulates from the plasma.

The above described problems of the conventional systems have beenovercome by the method and apparatus according to the present inventionwhich is grounded on the realization by the present inventors thatparticulates in a plasma follow the bulk flow of the ions in the plasma.

Generally, according to the particulate reduction method of theinvention, multipole magnetic fields are used for generating flows nearthe substrate (e.g., wafers) being processed. The inventors recognizethat such multipole magnetic fields may be incidentally present in someplasma tools, such as planar inductive tools, e.g., the Tegal MPC tool,commercially available from Tegal, Inc. However, even assuming thatthese magnetic fields are present, the fields are not controlled,regulated or tailored in any manner near the object (e.g., asemiconductor wafer) being processed, to produce the plasma flowsrequired for particulate reduction. This is a serious deficiency in thatcontaminants will be present on the wafer, thereby affecting itsperformance.

Plasma flow may also be produced by generating a plasma over the wafer,or using a diffusion plasma for producing plasma over the wafer, and bypreventing plasma generation adjacent the wafer and between the waferand a pumping aperture.

In view of the foregoing, according to a first embodiment of the presentinvention, magnetic fields are formed to produce a plasma flow which isdirected radially away from an object to be processed. The magneticfields are preferably formed by a plurality of multipole magnets.

According to a second embodiment of the present invention, magneticfields are formed by an array of magnets which is provided to produce anopening through which plasma will flow. The array of magnets includes aline cusp arrangement of magnets.

With the invention, in a plasma confined by a multipole magnetic field,the plasma is confined away from the multipole structure between themagnetic cusps and extends out toward the structure near the cusps.Normally, the plasma flow toward these cusps is minimal due to themirror effects of the cusp.

The present inventors have caused plasma flow through the pumpingapertures in two ways: 1) by essentially pulling a magnetic cusp apartso that its respective halves are on opposite sides of these apertures,thereby greatly reducing the magnetic field and thus reducing the mirroreffect of the weak cusp at the aperture; and 2) in another embodiment,the E×B plasma drift is used to cause the plasma to flow through theapertures.

Additionally, the plasma loss to the surface adjacent the wafer is usedto enhance the flow. This is performed by using the magnetic field abovethis area to reduce the volume of plasma which can flow to this surface.Thus, a flow transverse to this surface is established.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of preferredembodiments of the invention with reference to the drawings, in which:

FIG. 1(a) is a cross-sectional, schematic drawing of a magneticallyconfined plasma in a known structure;

FIG. 1(b) is a cross-sectional view taken along line II--II of FIG.1(a);

FIG. 2(a) illustrates a first embodiment according to the presentinvention;

FIG. 2(b) illustrates a modification of the embodiment in FIG. 2(a);

FIG. 3(a) illustrates a further modification of the invention shown inFIG. 2(a) in which magnetic fields are formed by a ring cusp arrangementof magnets to produce a plasma flow which is directed radially away froman object to be processed;

FIG. 3(b) illustrates a modification of the invention shown in FIG.3(a);

FIGS. 4(a) illustrates a second embodiment of the invention in whichmagnetic fields are formed by a line cusp array of magnets to produce anopening through which plasma will flow;

FIG. 4(b) is a plan view of a modification of the magnet configurationof the arrangement of FIG. 1(a), in which the poles are pulled apart andare turned to face each other and in which pumping apertures areinserted at the poles;

FIG. 5 illustrates a third embodiment of the invention in which plasmais produced by a radio-frequency (RF) electrode and in which fieldsproduced by the RF electrode between the wafer and the pumping apertureare reduced;

FIG. 6 illustrates the relationship of the plasma density and theassociated electrical fields which exist near the pumping aperture inthe structure illustrated in FIG. 1(a) and 1(b); and

FIG. 7 illustrates the relationship of the plasma density and theassociated electrical fields which exist near the pumping aperture inthe structure illustrated in FIG. 2(b) having a line cusp configuration.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1(a)-1(b),an apparatus for use with a first embodiment of the present invention isshown. Generally, the invention is for use in a dry processing apparatusfor plasma etching or deposition. An example of such a plasma tool isdisclosed in European Patent Publication No. 0379828A2, corresponding toU.S. patent application Ser. No. 07/301,933, to Coultas et al., assignedto the assignee of the present application and incorporated herein byreference.

The plasma processing apparatus 1 includes a cylindrical, evacuatedchamber 2 containing a workpiece 3 (e.g., a semiconductor wafer) fortreatment with a plasma process. A gas is admitted to chamber 2 from amanifold 4 via an orifice 5. The gas is used to form a plasma forprocessing of wafer 3 by etching or deposition in a manner known in theart. A liner 6, preferably comprising a material which is substantiallyinert or noncontaminating to the plasma to be contained in the chamber2, forms a cylindrical outer wall and contains the gas which is to beenergized to form a plasma. A cover dielectric window 7 is also providedas shown in FIG. 1(a) such that the chamber is substantially surroundedby the cover 7, liner 6 and wafer 3. Wafer 3 is supported by a base 8suitably configured to be a wafer electrode biased by an R.F. biasingsource 12. A ring 9 separates the plasma from the surfaces below andretains the wafer 3 in the chamber.

Gas is admitted into the chamber through a gas input port 10, a gasinput line 11, manifold 4 and an orifice 5. The gas to be exhausted fromthe chamber 2 passes through an orifice 13 at the base of liner 6 andmultiple-magnetic-dipoles (e.g., multipoles) 14 into an exhaust vacuumpumping manifold 15 and out through port 16.

The multipoles are arranged around the periphery of liner 6 as shown inFIG. 1(b) and have their magnetic fields directed inwardly. With thisarrangement, a wall of magnetic field is provided which repels electrons(e.g., "e" in FIG. 1(b)) back into the interior of the chamber 2,thereby increasing the uniformity of the plasma near the wafer 3.

As shown in FIG. 1(b), the magnetic field contains cusps 17 pointingtowards the multipoles 14. As is known in the art, a radio frequency(RF) coil (e.g., designated by reference numeral 18 in FIG. 1(a)) iswound on top of cover 7 and is energized by an RF source (unreferencedin FIG. 1(a) but shown in the right-hand side of coil 18 as viewed. Theother elements shown in FIG. 1(a) are not germane to the presentinvention and thus description thereof will be omitted.

The radio frequency (R.F.) induction field conventionally applied to thechamber 2, generates a plasma in the gas. The plasma is confined withinthe external wall in the chamber 2 by the magnetic dipoles providing asurface magnetic field for confining the plasma. The surface magneticfield is confined to the space adjacent to the external wall. An R.F.generator provides an R.F. generated bias to the workpiece. The chamberis lined with a material inert to a plasma or noncontaminating to theworkpiece, and the induction source in the form of a spiral inductioncoil or the like is located on the exterior of the liner material on theopposite side of the chamber from the workpiece. Delivery of anddistribution of the gas to the chamber is uniform about the periphery ofthe chamber and is provided by a manifold located about the periphery ofthe chamber. An orifice for admitting gas from the manifold to thechamber is formed around the periphery of the chamber.

Turning to FIG. 2(a), the structure of the first embodiment according tothe invention is illustrated, and is for use with a plasma processingapparatus similar to that shown in FIGS. 1(a)-1(b). Elements shown inFIG. 1(a)-1(b) which are not required for a full understanding of theinvention, have been omitted from FIG. 2(a) for the sake of clarity.

Examining the structure of the invention in more detail, a workpiece 21to be processed (e.g., a semiconductor wafer made of silicon or thelike) by the plasma 20 in the chamber (unillustrated) is held by aclamping member 22 which may be a mechanical clamp or an electrostaticclamp, both of which are suitably known in the art.

A plurality of multipole magnets 23, 23', 23", are suitably arranged onrespective portions of a wall member 24 to have a ring cuspconfiguration. The wall member 24 is structurally formed such that eachof the magnets are preferably separated by a predetermined distance(e.g., preferably slightly smaller than the distance between the magnetand the object) from each other and such that the strength of themagnetic fields decreases exponentially towards the wafer. The distancebetween adjacent ones of the magnets should be less than the distancebetween the wafer and the multipole magnet closest to the wafer. Themagnets preferably comprise rare earth metals-cobalt alloys, neodymiumiron boron, ceramic or aluminum/nickel alloys and may be embedded in orsimply positioned on the respective wall member portions.

The magnetic orientation of the multipole magnets is alternatedtherebetween such that south and north poles of a first magnet 23 facenorth and south poles of a second, adjacent magnet 23', respectively.Arc-shaped magnetic fields 25, 26 are generated between magnets 23 and23' and between magnets 23' and 23", respectively. A cusp 27 is formedat magnets 23 and 23'. A weak cusp 27' is also formed as shown in FIG.2(a). Preferably, the integral of the magnetic fields 25, 26 over thedistance perpendicular to the field should be large compared to the50-100 Gauss*cm range. For example, the magnetic fields preferably havea strength of approximately 1000 Gauss at the cusp of the plasma and2000 Gauss at the magnet face.

A magnet 28 is also provided such that the clamp is interposed betweenthe magnet 28 and the lowermost multipole magnet shown above the clamp,i.e., the third magnet 23" in the example shown in FIG. 2(a). The magnet28 may be formed as part of the underside of the clamping member or maybe formed separately therefrom and be clamped to the clamping member, ormay be part of a wall structure. The poles of the magnet 28 are opposedto like poles in the third magnet 23", so that an "opening" (e.g.,positioned at the weak cusp 27', as discussed in further detail below)is formed in the magnetic fields.

A pumping aperture 29 for pumping fluid (i.e., a neutral gas or thelike) is preferably provided substantially near the weak cusp or openingfor causing the gas to flow and entrained particulates to flow throughthe aperture from the chamber into an antechamber (e.g., similar tochamber 15 in FIG. 1(a)). The aperture 29 includes an orifice throughwhich the pumping fluid is pumped. The orifice is preferably positionedsubstantially at or near the opening. The neutral gas may be anelectronegative gas (e.g., a gas having a higher ion density than anelectron density) such as CF₄, CHF₃, Cl₂, HBr or O₂. The pump may have aplurality of different and/or variable pumping pressures foraccommodation of a plurality of different plasma types and differentsize contaminants and can be arranged with various different sensingmechanisms for sensing plasma flows.

The pump's orifice is suitably positioned adjacent the plasma flow andat a position below the multipole magnets at which the plasma flowstagnates. The plasma sheath prevents particulates from going throughit. The inventive system avoids sharp edges of the plasma in that adiffuse boundary is maintained in the plasma such that particle flowwill not be limited by a plasma sheath. Accordingly, when the plasmaslows down, the pumping of the fluid sweeps (or draws) the plasma flowand the particulates therein away from the wafer through the "hole" oropening in the field and out from the system.

As a result of such a structure, magnetic fields are formed having thelines of force illustrated. With such an orientation, the plasma flow isas shown and is positioned to be at an orifice of a mechanism forpumping fluid (i.e., a neutral gas or the like) such that as the plasmareaches a position below the multipole magnets, the plasma flow willstagnate. However, since the orifice of the pumping mechanism is locatedat the stagnation point of the plasma flow, the resulting flow ofneutral gas or the like from the pumping mechanism continues to sweepthe particulates past the stagnation point of the plasma flow, therebyeffectively preventing or reducing the contamination of the wafer beingprocessed. As mentioned above, such a method and apparatus are effectivein preventing contamination in objects having any size and in removingparticulates from the plasma, thereby rendering the object substantiallyfree of contaminating particulates.

FIG. 2(b) illustrates a modification of the embodiment shown in FIG.2(a) with a line cusp configuration but without the wafer clamp surfacebeing interposed between the weak cusp. As shown in FIG. 2(b), magnets23 and 23' are similar to that of FIG. 2(a). However, the magnet 30 hasa first surface directly opposed to the wafer clamp which has a singlepolarity (e.g., S in the illustrated example) arrayed on the surface. Asecond surface has an opposite polarity (e.g., N) arrayed thereon andfaces the second magnet 23'. Likewise, a magnet 30 is suitably arrangedbelow the wafer clamp such that the surface of magnet 31 facing asurface of magnet 30 has the same polarity thereto. An aperture forpumping fluid is provided similarly to the embodiment of FIG. 2(a). Thisarrangement is advantageous in producing less confinement of the plasmaflowing through the opening by virtue of the smaller magnetic fieldsperpendicular to the plasma flow. Further, as shown in FIG. 7 (describedin greater detail below) such a configuration produces less electricalconfinement of the plasma flow.

FIG. 3(a) illustrates a further modification of the embodiment shown inFIG. 2(a), in which the multipole magnets 50, 50' are mounted in astructure 51 and which can include pole pieces 52, 53 comprising iron orthe like formed thereon such that a relatively strong magnetic field canbe formed for confining the high energy electrons and the plasma awayfrom the wall area of the chamber. A relatively weak "pole" ispositioned near the pump aperture. A third magnet 50" is positioned withan iron pole piece as shown. The difference in magnetic strength betweenthe strong pole and the weak pole is preferably as great as possible.For example, the strong pole preferably has a strength of 500 Gauss andthe weak pole has a strength of less than 80 Gauss. As a result of thestrong pole-weak pole combination, plasma will flow to the wall areanear the pump aperture. An ion drag force will "pull" particulates outof the plasma to the region of the pump port where the neutral gas flowwill drag the particulates through the pump port and through the weakplasma on the distal side of the apertures. The strength of the polesmay be suitably adjusted to vary the magnetic field profile to achieve adesired profile for a particular plasma.

A similar effect may be achieved by using appropriate magneticmaterials, such as rare earth metals-cobalt alloys, neodymium ironboron, ceramic or aluminum/nickel alloys, or a combination of the abovemagnetic materials, in lieu of the pole pieces.

Hence, with the invention, as the plasma reaches a position below themultipole magnets and the plasma flow stagnates, the presence of thepumping orifice located at this position advantageously enables thepumping of the neutral gas to continue to sweep the particulates pastthe stagnation point of the plasma flow. Thus, the workpiece is reliablykept from being contaminated by particulates. In other words, theworkpiece is advantageously and reliably kept free of particulatecontamination during plasma processing.

Further, it is often beneficial to provide a magnetic bucket or surfacemagnetic field for confining energetic electrons in a particular area ofthe plasma. These surface magnetic fields are the most effective if thecusps are suitably long such as 5 cm. or the like and preferably areformed as ring cusps at the outer area of a cylindrical plasma.

However, if the plasma is for plasma processing, then it is alsobeneficial to reduce the magnetic field to about 5 Gauss or less at theradius of the outer edge of the wafer. Prior to the present invention,solutions to these requirements were mutually exclusive since in theregion in which the ring cusps stop, for example, at the plane of thewafer, the fields flare out toward the center of the plasma. However,with the present invention, such a solution has been found to satisfyboth requirements.

Specifically, the invention varies the strength of the ring cusp tooptimize the plasma processing. For example, the strength of the firstand last pole is reduced relatively to the other poles of the ringcusps. For example, the strength of the first pole and the last pole ispreferably 1/4 or less of the other poles of the ring cusp which mayhave a strength of more than 500 Gauss. Thus, the magnetic fields areoptimized for plasma processing.

FIG. 3(b) illustrates another configuration of the multipole magnets inwhich a multipole magnet 40 is positioned below the clamp. Magnet 40 hasan orientation which is rotated 90 degrees from that of the orientationof the magnet shown below the clamp in FIG. 3(a), such that both poles(e.g., N and S) of the magnet below the clamp face a single pole (e.g.,N) of the magnet 41 immediately above the clamp. This configuration isadvantageous in that better reduction in magnetic fields in the openingresults, thereby allowing the particles to move freely therethrough.

A second embodiment of the invention is shown in FIG. 4(a), in which asuitably arrayed magnetic configuration is used, along with lo plasmaconfinement by line cusps, to produce a plasma flow radially away from asubstrate being processed. The configuration is advantageous for "E×Bdrift", which (shown as reference numeral 73a in FIG. 4(a)) serves tochannel the flowing plasma through the opening. "E×B drift" is definedas the vector cross product of the electric field and the magnetic fluxand is further advantageous for causing both the electrons and the ionsto drift in the same direction. In this configuration, this drift causesa flow of the plasma toward the aperture. This plasma flow dragsparticulates along with it, away from the workpiece. The magnets 73,which are configured in a line cusp o arrangement, are suitably mountedin a wall member 74 surround the clamping member so as to produce an"opening" in the magnetic field through which plasma and entrainedparticulates flow, as illustrated by the arrows in FIG. 4(a).

Specifically, in FIG. 4a, the workpiece 71 (e.g., a semiconductor wafermade of silicon or the like) to be processed in a plasma chamber isclamped by the clamping member 72. A plurality of multipole magnets 73are vertically arranged in a line cusp manner on respective portions ofa wall member 74. Each of the magnets are preferably separated by apredetermined distance (e.g., such as 1.5 inches). In one example, thering cusp may be 2 inches from the wafer and the ring cusp may have adiameter of 11 inches (the cusp having 30 magnets being separated by 1.1inches). Thus, the magnets are preferably separated from each other,such that the strength of the magnetic fields falls off exponentially asthe magnetic fields near the plasma edge. The distance between adjacentmagnets should be less than the distance between the wafer and themagnets.

FIG. 4(b) illustrates a plan view of a modification of the magnetconfiguration of FIG. 1(a), in which the poles are pulled apart andturned to face each other and in which pumping apertures are inserted atthe poles. FIG. 4(b) illustrates the line cusp magnets facing each otherand the arrows represent the plasma being allowed to flow out betweenthe magnets. More specifically, plasma flows out from the chamber at thecusp 80 formed between adjacent magnetic fields of the magnets. Theadvantage of this line cusp arrangement is the ease of construction ascompared to the ring cusp arrangement shown in FIG. 2(b).

The magnets preferably comprise rare earth metals-cobalt alloys,neodymium iron boron, ceramic or aluminum/nickel alloys and may beembedded in or simply positioned on the respective wall member portions.

Similarly to the embodiments above, an aperture 75 for pumping fluid andhaving an orifice through which the fluid is pumped out, may beadvantageously provided. The orifice is suitably positioned adjacent theplasma flow, and, more particularly, at a position below the multipolemagnets proximate to the position where the plasma flow stagnates.Hence, the pumping of the fluid via aperture 75 sweeps the plasma flowand the particulates therein away.

As a result of such a structure, a channel is formed for the plasma toflow out from the chamber. With such an orientation, the plasma flow isas shown and is positioned to be at the orifice for pumping fluid suchthat as the plasma reaches a position below the multipole magnets andthe plasma flow will stagnate. However, since the pumping orifice islocated at the stagnation point, the resulting flow of neutral gas orthe like continues to sweep the particulates past the stagnation pointof the plasma flow, thereby effectively processing the workpiece andrendering the workpiece substantially free of undue particulatecontamination.

FIG. 5 illustrates a third embodiment of the invention, in which a wafer82 is securably held in contact to a wafer electrode 81. A plasma 89 isproduced over the top of the wafer 82. Preferably, the plasma isproduced by an RF coil 88 or the like. Any RF fields between the plasmaand ground electrodes 83, which are produced from RF voltage onelectrodes 81 and coil 88, are reduced by insulator 85. Likewise, anyfield between the plasma and the electrodes 83 near aperture 90 can bereduced as those to electrode 86 by isolation insulator 87. In this way,the production of the plasma in the region between the wafer and pumpingaperture 90 is reduced. Thus, the plasma flows into this region andeasily out of aperture 90. Preferably, the boundary of the plasma 91outside of the aperture 90 should be diffuse so that particulates,entrained in this plasma flow, can easily flow through aperture 90. Inthe region near aperture 90, the particulates are also entrained in aneutral gas flow across boundary 91 and are pumped away from the waferand out of the aperture.

FIG. 6 illustrates the relationship of the relative plasma density(e.g., represented by the broken line and labelled " relative plasmadensity) and the floating potential (e.g., represented by the solid lineand labelled "floating potential"), near the pumping aperture, to theradius R of the magnetic configuration of the known system shown inFIGS. 1(a)-1(b). Since for a flowing plasma, dV/dx-KT_(e) (dN/dx)/N andKT_(e) is approximately 6 eV, the small changes in the floatingpotential between R=0 and 10 cm., as shown in FIG. 6, indicates that theplasma is not flowing toward the cusp. Thus, particulates will not bedragged toward the pumping aperture. For 10<R<22 cm, there is a largepeak in the floating voltage which indicates that a deep particle trapexists near the peak voltage. Further, the bucket edges are as shown inFIG. 6.

FIG. 7 illustrates the relationship of plasma density and electricalfields to radius R of the magnet configuration near the pumpingaperture, for the system having a line cusp arrangement similar to theconfigurations of FIGS. 2(b) and 4(b) according to the invention.

In contrast to the graph of FIG. 6, in FIG. 7 there is a much largervoltage variation between 0<R<17 cm. This larger voltage variationindicates that the positive ions are being pulled toward the weak cuspby this field. Thus, the plasma is flowing toward the cusp and the iondrag force drags particulates toward this cusp and the associatedpumping aperture. Further, the voltage peak between 17<R<20 cm. is muchsmaller in FIG. 7 than that of FIG. 6 and is primarily inside theaperture. Thus, the plasma flow and gas flow will prevent theparticulates from being trapped.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. In a plasma processing systemused in manufacturing an object in which the object positioned in ahousing is exposed to a plasma discharge, an apparatus for preventingparticulate from contaminating the object, comprising:magnetic means forgenerating a plasma flow adjacent the object, said plasma flow flowingtoward at least one pumping aperture; pumping means for pumping amedium, said medium supporting said plasma discharge and entraining saidparticulate in said plasma flow; and wall confining means beingpositioned adjacent said at least one pumping aperture, said magneticmeans for producing a first cusp having a first strength at said atleast one pumping aperture, said magnetic means comprising a pluralityof magnets adjacent said at least one pumping aperture on a sideopposite said object, each magnet of said plurality of magnets having asecond cusp having a second strength, said first strength having a valueless than that of said second strength.
 2. The apparatus according toclaim 1, wherein:a magnet of said plurality of magnets which is closerto a pumping aperture of said at least one pumping aperture than othermagnets of said plurality of magnets, having a strength less than astrength, of said other magnets, such that an absolute magneticpotential of said magnet having said strength less than said adjacentmagnet is no more than half of that of the other magnets of saidplurality of magnets.
 3. The apparatus according to claim 2, whereinsaid magnetic means includes a line cusp of magnets for generating theplasma flow radially away from the object.
 4. The apparatus according toclaim 2, wherein said magnetic means includes a ring cusp of magnets forgenerating the plasma flow radially away from the object.
 5. Theapparatus according to claim 2, further comprising a second magnetpositioned across said pumping aperture opposing said magnet.
 6. Theapparatus according to claim 5, wherein said magnet has an end having afirst polarity facing said pumping aperture and said second magnethaving an end having said first polarity facing said pumping aperture,thereby creating said first cusp.
 7. An apparatus according to claim 5,wherein said magnet has a positive end facing said pumping aperture andsaid second magnet having a positive end facing said pumping aperture,thereby creating said first cusp.
 8. The apparatus according to claim 5,wherein said magnet lies perpendicular to said pumping aperture and saidsecond magnet lies perpendicular to said pumping aperture so that apositive end of said magnet lies in a first direction and a positive endof said second magnet lies in said first direction, thereby creatingsaid first cusp.
 9. In a plasma processing system used in manufacturingan object in which the object positioned in a housing is exposed to aplasma discharge, an apparatus for preventing particulate fromcontaminating the object, comprising:means, operatively coupled to saidhousing, for generating a plasma flow adjacent the object toward apumping aperture; and means for pumping a medium adjacent the object,said medium supporting said plasma discharge and entraining saidparticulate in said plasma flow away from the object, wherein said meansfor generating said plasma flow comprises means for generating saidplasma discharge and radio frequency isolation means for eliminatingplasma generation in a region adjacent said pumping aperture such thatproduction of said plasma discharge between said object and said atleast one pumping aperture is reduced and wherein a boundary of theplasma discharge in a region of said at least one pumping aperture isdiffuse.
 10. An apparatus according to claim 9, wherein said means forgenerating said plasma flow comprises means for inductively generatingan electric field in said plasma.
 11. In a plasma processing system usedin manufacturing semiconductor circuits, wherein a semiconductor waferis mounted on a clamping member with a chamber and is exposed to a radiofrequency generated plasma discharge in the chamber, an apparatus forpreventing particulate deposition on the wafer, comprising:a housing;magnetic means, mounted on said housing, for generating a plasma flowadjacent the wafer toward at least one aperture; and means for blowingparticulates in the radio frequency generated plasma discharge above thewafer past a first point of said plasma flow, wherein said means forblowing includes means for pumping a gas flow for entrainment of theparticulates through said at least one aperture, said magnetic meansproducing a first cusp having a first strength at said at least onepumping aperture, said magnetic means comprising a plurality of magnetsadjacent said at least one pumping aperture on a side opposite saidwafer, said magnetic means eliminating radio frequency generated plasmadischarge generation in a region adjacent said at least one aperturesuch that production of said radio frequency generated plasma dischargebetween said wafer and said at least one aperture is reduced, eachmagnet of aid plurality of magnets having a second cusp having a secondstrength, said first strength having a value less than that of saidsecond strength, a boundary of the radio frequency generated plasmadischarge in a region of said at least one aperture being diffuse. 12.An apparatus according to claim 11, wherein said magnetic means includesa line cusp of magnets for generating a plasma flow radially away fromthe wafer.
 13. In a plasma processing system used in manufacturingsemiconductor circuits, wherein a semiconductor wafer is mounted on aclamping member with a chamber and is exposed to a radio frequencygenerated plasma discharge in the chamber, an apparatus for preventingparticulate deposition on the wafer, comprising:a housing; means,mounted on said housing, for generating a plasma flow adjacent the wafertoward an aperture; and means for blowing said particulate on the waferpast a first point of said plasma flow, wherein said means for blowingincludes means for pumping a gas flow for entrainment of saidparticulate through said aperture, wherein said means for generating aplasma flow includes means for generating a magnetic field, wherein saidmeans for generating a magnetic field includes a line cusp of magnetsfor generating a plasma flow radially away from the wafer producing afirst cusp having a first strength at said aperture, wherein each magnetof said line cusp of magnets has a second cusp having a second strength,said first strength having a value less than that of aid secondstrength.
 14. In a plasma processing system used in manufacturingsemiconductor circuits, wherein a semiconductor wafer is mounted on aclamping member with a chamber and is exposed to a radio frequencygenerated plasma discharge in the chamber, an apparatus for preventingparticulate deposition on the wafer, comprising:a housing; means,mounted on a housing, for generating a plasma flow adjacent the wafertoward an aperture; and means for blowing said particulate in the radiofrequency generated plasma discharge above the wafer past a first pointof said plasma flow, wherein said means for blowing includes means forpumping a gas flow for entrainment of said particulate through saidaperture, wherein said aperture is positioned at a region of saidhousing at which said plasma flow has a zero velocity.